1. Field of the Invention
The present invention relates to a liquid crystal optical element and a method for fabricating the liquid crystal optical element. More particularly, the present invention relates to a liquid crystal optical element for use in a display device for presenting characters, graphics and so on, and an optical shutter for changing the quantity of incoming light transmitted, and a method for fabricating such a liquid crystal optical element.
2. Description of the Related Art
Recently, liquid crystal displays (LCDs) have found a broad variety of applications as displays for TVs, computers, mobile electronic units and so on by utilizing their advantageous features including significantly reduced size or weight and power dissipation.
However, the conventional twisted nematic (TN) mode LCDs and super twisted nematic (STN) mode LCDs achieve narrow viewing angles. To overcome this problem, various “in-plane switching modes” were proposed as modes to control the orientation directions of liquid crystal molecules in a liquid crystal layer.
For example, in a proposed in-plane switching mode for liquid crystal molecules, a lateral electric field is generated by comb-shaped electrodes parallel to the surface of a nematic liquid crystal layer. According to other proposed modes, ferroelectric and antiferroelectric liquid crystal layers are also used. In each of these proposed modes, the liquid crystal molecules change their orientation directions parallel to the surface of the liquid crystal layer, thus achieving a wide viewing angle characteristic.
In the mode of generating the lateral electric field, however, no electric field is generated over the comb-shaped electrodes and portions of the liquid crystal layer, located over those electrodes, do not contribute to display operation, thus resulting in a significantly decreased aperture ratio. To generate the lateral electric field, at least two electrodes need to be provided within each picture element region such that different potentials are applied to two adjacent electrodes. Normally, several electrodes are densely arranged at a shortened pitch so as to generate a sufficiently strong lateral electric field. However, only the gaps between those electrodes can respond to the applied voltage and contribute to the display operation. In this mode, even if the electrodes are made of a transparent material, no electric field is generated over the electrodes, either, and portions of the liquid crystal layer over the electrodes never respond to the applied voltage or contribute to the display operation. Consequently, considering its operating principle, this mode utilizing the lateral electric field should result in a lower aperture ratio than a mode utilizing a vertical electric field.
On the other hand, in the mode utilizing the ferroelectric or antiferroelectric liquid crystal layer, the cell thickness must be about 2 μm or less. At such a small cell thickness, the contrast ratio should decrease unless defects are totally eliminated from the orientation state of the liquid crystal molecules. Thus, such a mode is hard to realize due to the difficulty of the manufacturing process. That is to say, LCDs operating in such a mode are difficult to produce constantly.
To overcome these problems, PCT International Publication No. 00/03288 discloses a liquid crystal optical element that includes: a liquid crystal layer made of a nematic liquid crystal material; and two more liquid crystal layers, which are provided so as to interpose the nematic liquid crystal layer between them and made of a ferroelectric liquid crystal material. FIG. 4 shows a liquid crystal optical element 1000 disclosed in the document identified above.
As shown in FIG. 4, the liquid crystal optical element 1000 includes a first substrate 1010, a second substrate 1020, and first, second and third liquid crystal layers 1016, 1030 and 1026 provided between the first and second substrates 1010 and 1020.
A first electrode 1012 and a second electrode 1022 are provided on the first and second substrates 1010 and 1020, respectively, so as to face each other with the first, second and third liquid crystal layers 1016, 1030 and 1026 interposed between them. A first alignment layer 1014 and a second alignment layer 1024 are provided so as to cover the first and second electrodes 1012 and 1022, respectively. Also, a first polarizer 1018 and a second polarizer 1028 are further provided on the outside surfaces of the first and second substrates 1010 and 1020, respectively.
The first and third liquid crystal layers 1016 and 1026 are located on the first and second alignment layers 1014 and 1024, respectively, and the second liquid crystal layer 1030 is provided between the first and third liquid crystal layers 1016 and 1026. The first and third liquid crystal layers 1016 and 1026 are made of a ferroelectric polymer liquid crystal material, while the second liquid crystal layer 1030 is made of a nematic liquid crystal material.
Hereinafter, it will be described with reference to FIGS. 5A and 5B how this liquid crystal optical element 1000 operates. Specifically, FIG. 5A shows a state of the liquid crystal optical element 1000 in which a predetermined voltage is applied between the first and second electrodes 1012 and 1022, while FIG. 5B shows another state of the liquid crystal optical element 1000 in which a voltage having the opposite polarity is applied there.
In the liquid crystal optical element 1000, a liquid crystal molecule 1016a included in the first liquid crystal layer 1016 and a liquid crystal molecule 1026a included in the third liquid crystal layer 1026 make in-plane switching parallel to the surfaces of the second liquid crystal layer 1030 (i.e., parallel to the inner surfaces of the first and second substrates 1010 and 1020) in response to the voltage applied between the first and second electrodes 1012 and 1022 as shown in FIGS. 5A and 5B. On the other hand, liquid crystal molecules 1030a included in the second liquid crystal layer 1030 between the first and third liquid crystal layers 1016 and 1026 also make in-plane switching under the influence of the liquid crystal molecules 1016a and 1026a in the first and third liquid crystal layers 1016 and 1026.
In the liquid crystal optical element 1000, the liquid crystal molecules 1030a make in-plane switching in this manner. Accordingly, a wide viewing angle characteristic is achievable by applying this liquid crystal optical element 1000 to a display device. In addition, the first and second electrodes 1012 and 1022 can be transparent electrodes, thus achieving a high aperture ratio. Furthermore, in this liquid crystal optical element 1000, there is no need to decrease the cell thickness excessively, and therefore, constraints on the manufacturing process can be relaxed. As a result, such a liquid crystal optical element can be produced easily enough.
The liquid crystal optical element 1000 may be fabricated in the following manner, for example.
First, a first electrode 1012, made of transparent and conductive ITO, is defined on a first substrate 1010 of glass, for example. Next, a first alignment layer 1014 of SiOx is deposited over the first electrode 1012. Subsequently, a first liquid crystal layer 1016 of a photopolymerizable liquid crystal material is provided on the first alignment layer 1014.
In the meantime, a second electrode 1022, a second alignment layer 1024 and a third liquid crystal layer 1026 are formed in this order on a second substrate 1020 in the same way.
Thereafter, the first and second substrates 1010 and 1020 are bonded together such that the first and third liquid crystal layers 1016 and 1026 on the first and second substrates 1010 and 1020 face each other. Finally, a liquid crystal material is injected within a vacuum into the gap between the first and second substrates 1010 and 1020, thereby defining a second liquid crystal layer 1030.
However, the liquid crystal optical element 1000 shown in FIGS. 4, 5A and 5B causes an unwanted coloring phenomenon when the display thereof is viewed obliquely (to a normal which is defined perpendicularly to the principal surface of the first or second substrate). Specifically, when the display is viewed along the major axis of the liquid crystal molecule 1016a as shown in FIG. 6A, then the outgoing light ray becomes bluish. On the other hand, when the display is viewed along the minor axis of the liquid crystal molecule 1016a as shown in FIG. 6B, then the outgoing light ray becomes yellowish. That is to say, in this liquid crystal optical element 1000, every light ray passing obliquely through the second liquid crystal layer 1030 (i.e., so as to form a tilt angle with respect to a normal which is defined perpendicularly to the liquid crystal layer 1030) becomes bluish or yellowish unintentionally. This is because the retardation of the liquid crystal molecule has wavelength dispersiveness (or wavelength dependence).
Also, in the manufacturing process of the liquid crystal optical element 1000 as disclosed in the document identified above, after the first and third liquid crystal layers 1016 and 1026 have been formed on the first and second substrates 1010 and 1020, respectively, the first and second substrates 1010 and 1020 are bonded together, and then the liquid crystal material of the second liquid crystal layer 1030 is injected. Accordingly, during such a manufacturing process, the first and third liquid crystal layers 1016 and 1026 are exposed to the air and may have disturbed orientation states. More specifically, in the vicinity of the interface between the liquid crystal layer and the air, the liquid crystal molecules turn a hydrophobic group toward the air and a hydrophilic group toward the depth of the liquid crystal layer, which is analogous in principle to soap bubble forming. As a result, the liquid crystal molecules are oriented vertically. Then, some of the liquid crystal molecules in the second liquid crystal layer 1030 are also oriented vertically under the influence of the first and third liquid crystal layers 1016 and 1026. Consequently, those portions make the screen brightness uneven, thus deteriorating the display quality.